In recent years, demands for high sensitivity, excellent graininess, gradation, high sharpness and excellent storage stability in silver halide photographic materials and, further, for rapid processing accelerating rate of development and the like have increasingly become severe. In particular, demands for good storage stability while suppressing fog in low level and further for higher sensitization have been strong.
A silver halide emulsion for use in a silver halide photographic material is, in general, chemically sensitized with various chemical substances for obtaining desired sensitivity and gradation.
Specifically, chalcogen sensitization such as sulfur sensitization, selenium sensitization and tellurium sensitization, noble metal sensitization using noble metals, such as gold, and reduction sensitization using reducing agents are used alone or in combination.
Of these sensitizing methods, that which is most widely and generally used and fundamental in combined sensitizing methods is a sulfur sensitizing method using labile sulfur compounds, and specific examples thereof are disclosed in P. Glafkides, Chimie et Physique Photoqraphigue, 5th Ed., Paul Montel (1987), T. H. James, The Theory of the Photographic Process, 4th Ed., Macmillan (1977), H. Frieser, Die Grundlaqen der Photoqraphischen Prozesse mit Silver haloqeniden, Akademische Verlagsgesellshaft (1968), U.S. Pat. Nos. 1,574,944, 1,623,449, 2,278,947, 2,410,689, 2,440,206, 2,449,153, 2,728,668, 3,189,458, 3,501,313, 3,656,955, 4,030,928, 4,054,457, 4,067,740, 4,266,018, 4,810,626, German Patents 1,422,869, 1,572,260, 228,658, 235,929, British Patents 1,129,356, 99,701, 1,403,980, European Patents 61,446, 138,622, JP-A-63-5355 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-63-5336, JP-A-63-229449, JP-A-58-80634, JP-A-1-114839, JP-A-1-227140, JP-A-5-165135, JP-B-58-30570 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-60-24457, JP-B-62-17216 and Research Disclosure, Vol. 307, No. 307105.
In recent years, high sensitivity while maintaining excellent graininess and high sharpness of silver halide photographic materials, further, high temperature rapid processing for increasing the processing rate and higher sensitization in high temperature forced development (by prolonging the processing time) have been strongly desired, however, by which the generation of fog and soft gradation are concurrently liable to be enlarged. Accordingly, the improvements of the above-described sulfur sensitization and combined sensitization with a sulfur sensitizing method as a fundamental such as a sulfur-gold sensitizing method and a sulfur-selenium-gold sensitizing method have been desired.
Silver halide photographic emulsions are, in general, spectrally sensitized with sensitizing dyes to have photographic sensitivity in wavelength regions such as green, red and infrared regions where silver halide per se does not have absorption.
The increase of the amount of sensitizing dyes used to increase the spectral sensitivity is often conducted, which, at the same time, causes the increase in reduction of the sensitivity in the intrinsic wavelength region, that is, the intrinsic desensitization, presumably due to development inhibition by the dyes, latent image dispersion, or nullification of photoelectron by the hole of the dyes and the bleaching of latent images or the like. As a result, the spectral sensitivity reaches a top limit and does not increase further.
In addition, the sensitivity fluctuation becomes large (mainly desensitization) during storage with the lapse of time by the use of large amount of dyes. Heretofore, various efforts have been done to still further increase the spectral sensitivity stably but yet insufficient.
In particular, the development of useful and universal techniques of chemical sensitization has been eagerly demanded to reduce the intrinsic desensitization by sensitizing dyes and to improve spectral sensitivity stably.